It has been proposed that the thermostat valve control acting on the by-pass line because of its arrangement also be simultaneously employed to control the heating means. It has been found, however, that conventional thermostat valves do not function with sufficient precision, so that the by-pass line is not shut off completely when the selected cooling water temperature is reached. Consequently, the heating means may be traversed to a slight extent by heated cooling water. In that case, the intended shut-off will be at least in part ineffectual, since the fresh gases being supplied to the engine continue to be heated, though not to any high degree.